In a magneto-optical recording medium such as a magneto-optical disk, the diameter of a light spot formed on the recording medium by a light beam for use in recording and reproduction is the same as that of a recorded bit; therefore, its recording density is restricted by the diameter of the light spot. Recently, however, in order to increase the recording density of an optical memory device, a magneto-optical recording medium has been proposed for reproducing recorded bits with diameters smaller than the diameter of a light spot.
Normally, the light beam for use in optical recording and reproduction is converged to a diffraction limit by a converging lens; therefore, the light intensity distribution shows a Gaussian distribution, and thus the temperature distribution on the recording medium also exhibits a virtual Gaussian distribution. As a result, a spot having a temperature that is not less than a predetermined temperature become smaller in size than the size of the light spot. Consequently, the recording density can be greatly improved if only the spot having a temperature not less than the predetermined temperature is utilized for reproduction.
Referring to FIG. 6, the following description will discuss a magneto-optical disk magneto-optical recording medium) wherein a recorded bit with a size smaller than the size of a light spot can be reproduced. The magneto-optical disk is mainly constituted by a readout layer 6 and a recording layer 7 that are formed on one surface of a substrate 5. The recording layer 7 has a great coercive force at room temperature. On the contrary, the readout layer 6 has a small coercive force at room temperature. When the temperature of an area of the readout layer 6 to be reproduced is raised, the direction of magnetization thereon becomes coincident with the direction of magnetization of the recording layer 7 due to the effect of the recording layer 7. That is, the magnetization of the recording layer 7 is copied onto the readout layer 6 by an exchange coupling force between the readout layer 6 and the recording layer 7.
In the above arrangement, information is recorded onto the magneto-optical disk by the ordinary photo-thermomagnetic recording method. When information is reproduced from the magneto-optical disk, it is necessary to make the direction of magnetization of the readout layer 6 coincident with a predetermined direction upward in FIG. 6) by applying an auxiliary magnetic field from an auxiliary magnetic field generating device 10 (Initialization). Then, a reproduction-use light beam 9 is projected onto the magneto-optical disk through a converging lens 8. Thus, the temperature of the readout layer 6 is locally raised at a center portion of a light spot and the magnetized information on the recording layer 7 is copied onto the readout layer 6. Thus, in this magneto-optical disk, only the information located at an area within the center portion of the light spot which has been irradiated by the reproduction-use light beam 9 and undergone a temperature rise above a predetermined temperature, is reproduced. Accordingly, recorded bits with a diameter smaller than that of the light spot of the reproduction-use light beam 9 are permitted to be read out.
However, in the above-mentioned conventional magneto-optical recording medium, prior to a reproducing operation, an auxiliary magnetic field needs to be applied from the auxiliary magnetic field generating device 10 for the purpose of initialization.
Further, a recorded bit on the readout layer 6 having the magnetized information which has been copied from the recording layer 7 in the reproducing operation remains as it is even after the reproducing operation has finished and the temperature has dropped. For this reason, when the irradiated area of the reproduction-use light beam 9 is shifted so as to reproduce the next recorded bit, the recorded bit having the residual magnetized information previously copied still exists within the light spot of the reproduction-use light beam 9, and this recorded bit having the residual magnetized information, which has been already reproduced, tends to be reproduced together with a signal from an aimed recorded bit during reproduction. This results in noise and thereby presents a problem in improving the recording density.
Furthermore, in the conventional magneto-optical recording medium, at least two types of magnetic layers, that is, the readout layer 6 and the recording layer 7, are required; this necessitates a complicated manufacturing process.